Optical flame detectors are old in the art of providing automatic detection of fires. A feature shared by all such optical detectors is a shield window so that dust, soot or oil cannot be directly deposited on the optical detectors. Optical detectors are known to provide broad or narrow frequency detection of infrared and/or ultraviolet range frequencies. For instance, a typical hydrocarbon fire will typically have detectable peaks in the wavelengths of 2.7 and 4.3 micrometers. Ultraviolet radiation, though typically emitted at low levels, is detectable by way of on optical detector for an appropriate frequency range.
A standard feature of present day flame detectors is a “self-test”, which includes a measurement of radiation of some type through the shield window to determine if dirt, soot, or oil has obscured passage of radiation through the shield window so substantially that a user must be notified that the flame detector is likely not be able to detect a flame. Demonstrating a common feature of prior art flame detectors is the self-test method shown in U.S. Pat. No. 7,956,329, which is incorporated herein. The '329 patent shows the use of a light source adjacent to the optical detectors of a flame detector that is transmitted through the shield window to a reflection device outside of the shield window, where the reflection device reflects the beam of light from the light source to an optical detector behind the shield window. The comparison of a previously detected clean window condition and a current window condition measured by reflected light from the light source impinging on the optical detector will produce a measurable difference if the shield window is not clean. The result of the determination that the shield window is not clean is converted to a signal or warning of some type so that action can be taken to clean the shield window.
The light source used in the above determination is typically in the infrared range so that the infrared detector(s) of the flame detector can either be directly used to measure the reflected light from the light source or some other detector can be used so that actual transmission of infrared range light through the shield window is measured, resulting in the measurement of obscuring of the shield window to the desired flame detection frequencies possible.
However, all the prior art self-test devices suffer from a common disability. They cannot measure obscuring of the field of detection of the flame detector of anything other than materials or objects on the outside surface of the shield window. There is a need for a flame detector having the ability to detect and report to a user or cause the flame detection system to take some action if the field of vision of the optical detectors is obscured by either of materials or objects on the outside surface of the shield window or for materials or objects which are removed at a determinable distance from the outside surface of the shield window.